Ultrahigh-temperature granulites from the southern margin of the Madurai Block, India: Petrology, metamorphic phase equilibria and zircon-monazite U–Pb geochronology

Abstract

The Madurai Block is the largest crustal block in the Southern Granulite Terrane of India, and is composed of a dominantly Neoarchean northern segment, Paleoproterozoic central segment, and dominantly Neoproterozoic southern segment. Here we investigate garnet – orthopyroxene – spinel – cordierite – plagioclase – K-feldspar – quartz bearing granulites from the southern margin of the Madurai Block. We present results from petrology, mineral chemistry, pseudosection modeling, geothermobarometry, and U–Pb geochronology of zircon and monazite. The prograde minerals assemblages comprise Grt + Pl + Qtz + Mt. + Ilm + Sill which define pressure-temperature (P-T) conditions of 7.2–7.8 kbar and 900–980 °C. The peak minerals assemblages of Grt + Pl + Opx + Crd + Qtz + Mt. + Ilm yield P-T conditions of 5.8–6.5 kbar and 980–1030 °C. Subsequent cooling and decompression are inferred by the break down of garnet at 2–4 kbar and < 850 °C. Detrital zircon grains in the garnet-orthopyroxene-cordierite- bearing granulite samples show similar age distributions and can be divided into magmatic cores (1047–600 Ma with age peaks at 869 Ma and 609 Ma) and metamorphic overgrowths (623–511 Ma), representing the early Neoproterozoic detrital source and Late Neoproterozoic - Cambrian ultrahigh-temperature metamorphism. Monazite shows a prominent age peak at 543 Ma with a few early Neoproterozoic detrital grains that show Pb loss associated with the Cambrian metamorphic event. The Cambrian ultrahigh-temperature metamorphism as recorded in our samples is similar to the reports from several other localities in the Southern Granulite Terrane and coincides with the tectonics associated with the final assembly and the Gondwana supercontinent. The common overprinting of the Cambrian UHT event at the junction of assembly of the East Gondwana fragments suggest mantle upwelling beneath this wide region, and possible heat input through underplated magmas.